1. Percentage Composition
Law of Definite Proportions
Percent Composition
Vitamin C application
Percentage Composition from Chemical Formula
Empirical Formula from % Composition
Molecular Formula
C2H6O

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3. Law of Definite Proportions
Over 200 years ago, scientists discovered that compounds contained elements in fixed mass proportions.
For example, in 2009 scientists discovered traces of water on the moon. They found this water had the same composition as water on Earth.
MH2O = g/mol
Looking at moles, 1 mol of water then contains 2.02 g of hydrogen and g of oxygen.
_______________________________________________ _______________________________________________ _______________________________________________

4. Law of Definite Proportions con’t...
We can calculate the mass percent of a compound using its chemical formula. Ex: Find the mass percent of hydrogen in H2O mass percent of H = mass of H x 100% mass of H2O

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6. Did You Know...
Humans can’t synthesize vitamin C and therefore must consume it in our diet. However, this wasn’t always possible until 1933 when scientists used percentage composition to synthetically produce vitamin C.

7. Percentage Composition from Chemical Formula
We can also determine percentage composition of a compound from its chemical formula and molar masses.
n is the number of moles of the element in 1 mole of the compound (its subscript).

8. Try This: Determine the percentage composition of C2H6O: C2H6O %C =
2 x (12.01 g)
46.07 g
x 100% = ______
%H =
6 x (1.01 g)
46.07 g
x 100% = ______
%O =
1 x (16.00 g)
46.07 g
x 100% = ______
Check:

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10. What is the chemical formula for a compound with the following elemental analysis?
49.5% C 5.2% H 28.8% N 16.5% O
Step 1: Determine the number of moles for each element
MC = 12 g/mol
Do the same for ……………………

11. nC = _________
nH = _________
nN = _________
nO = _________
Step 2: Determine the mole ratios
To determine this you divide everything by the lowest number of moles
4.125 mol = mol = 4 mol
1.03 mol
C:H:N:O  ______
The chemical formula is ________

12. Emperical Formula A formula ………………… When doing this:
Always best to assume that 100g of a sample is present if given the percent composition.
_____________________________
Determine lowest possible mole ratio by dividing by smallest mole value

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14. We need to have a whole number ratio, therefore we do so by multiplying ____________ by a number that will make them all whole.
Decimal #
Multiply by
Result
.85 and above
N/A
round up .8 5
Whole
.75 4
.66 3 .5 2
.33
.25
.20
.15 or less
round down
Mole Ratio
4.86/2.08 = 2.34
8.33/2.08 = 4
2.08/ = 1

15. Check Your Understanding
Calculate the empirical formula for the compound containing: C: 60.0%, H: 4.48%, O: 35.53%

16. Molecular Formula
The formula for a compound that shows the number of atoms of each element that make up a molecule of that compound.
When determining the molecular formula, the molar mass of ____________________________________
____________
Dividing the molecular formula molar mass by the empirical formula molar mass determines a multiple that can be applied to the empirical formula.

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18. Check Your Understanding
A compound with a molar mass of 180 g/mol is found to be composed of 40% carbon, 6.65% Hydrogen and 53.3% Oxygen. Determine the molecular formula.

19. Putting it all together
Determine the molecular formula of an unknown compound containing 45.7 % Si, 3.7 % H, 11.4 % N, and sulfur. The molar mass of the compound was determined to be g/mol
Element
Mass
Moles
Mole Ratio EF Si
45.7 H
3.7 N
11.4 S

20. Putting it all together
MEF = _____________
MMF = g/mol
Ratio = MMF = =
MEF
Molecular Formula: ___ x Si4H9N2S3 = ___________

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22. Hydrates

23. Hydrates
A hydrate is a compound containing a specific number of ___________
Most hydrates are have a ___________
The loosely bound water molecules can be removed which would then alter their structure.
The bound water does not affect the ___________

24. If you are trying to identify an unknown sample in a lab chances are that no one will supply you with the empirical formula and molar mass.
You are ………………..
You can do this by _________________a known mass (water) by ……………………
There are different procedures for different compounds.

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26. Analysis of a Hydrated Salt
We need to know this information because the ___________
The simplest way to determine the amount of water in hydrates is to heat a known mass to release its water. You then determine the mass of the anhydrous salt that ___________

27. Analysis of a Hydrated Salt
Example: A 100 g sample of Ba(OH)2xH2O(s) contains 54 g of Ba(OH)2(s). What is the value of x? Step 1: Calculate the amount in moles of barium hydroxide in the sample. Step 2: Find the amount in moles of water in the sample.

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